Direction finders are old and well known in the art and are used to determine the direction from which a transmitted signal is being received. The equipment varies in size, but it would be desirable to employ a small, wrist-worn direction finding receiver that would not interfere with the activities of the user. The size of such unit is limited by the circuitry and the size and spacing of the antennas required.
Such receivers in the past have used three antennae, two loops (a null antenna and a sense antenna) and an omnidirectional antenna. The signal from the null loop antenna mounted in the forward end of the receiver is compared with that of the sense loop antenna, which is mounted orthogonally, to determine that a null of some predetermined depth is being obtained. The phase of the omnidirectional antenna is compared with the phase of the sense loop antenna to determine which null is being encountered. If the relative phase is 0.degree., the direction lamp is illuminated. If the relative phase is 180.degree., the lamp is not illuminated.
This system depends upon having three antennae whose amplitude and phase characteristics are mutually independent from one another and are acceptable in the presence of the user. This means that the antennae must not be too close to one another or to the body of the user. Further, in the prior art, the omnidirectional or sense antenna must be mounted outside the receiver case in order to have sufficient spacing from the arm of the user and from the receiver to maintain an acceptable constant phase pattern. This presents an operational problem in that the omnidirectional antenna protrudes sufficiently to cause a hazard for the user. In addition, the omnidirectional antenna structure is subject to damage because of its protrusion from the receiver and has been one of the major causes of receiver failure.
Other direction finding techniques require antenna systems with large antennae, large spacings or both. Examples are dipoles that are spaced approximately 1/4 wave length apart and switched arrays of similar antennae.
These systems are too large to be worn on the arm of an operator. They depend on antennae which have antenna patterns which are mutually independent, limiting the minimum spacing and size which may be obtained. In addition, the three antenna approach requires means for determining which null (forward or back) is being encountered by measuring relative phase. This means that additional circuitry is required.
The present invention overcomes the disadvantages of the prior art by its miniature size and uses only two antennae instead of three. Further, the location and size of the omnidirectional antenna are not critical, resulting in the ability to contain the antenna within the package and resulting in a much smaller receiver design. Operation is very simple, allowing the user to concentrate on other tasks at hand and allowing an unambiguous direction indication. The size and weight of the equipment described is less than one-half that of the known prior art and the circuitry is less complex. The equipment is contained in a small package measuring approximately 2.7.times.5.times.0.9 inches resulting in a volume of approximately 12 cubic inches. This package can be worn on the arm of a user and has no protruding antennae. Reliability of the equipment is enhanced since the protruding antenna that was subject to breakage is now contained within the receiver and is therefore protected.
Thus the present invention results in a reduction in size and complexity of a miniature direction finder system by eliminating the need for one of the null loop antennae and allowing the use of a less critical omnidirectional sense antenna. In addition, the overall circuitry required by the receiver is less complex.
Further, the receiver sensitivity of the present invention has been increased by the use of a more efficient electrically tuned loop antenna resulting in a decrease of the required transmitter power to obtain the same range performance.
The ability of the present invention to use only a single null loop antenna for direction finding is significant in that the physical volume of the receiver can be reduced to approximately one-half the physical volume of the prior art receivers. The cost of the receiver circuits are also less since no ambiguity circuits are required to determine which null is correct. Reliability of the receiver is enhanced because there is no external omnidirectional antenna and because critical phase adjustments required for correct operation of the ambiguity circuits are eliminated.